Tony Heald / RF- Hippopotamus (Hippopotamus amphibius) surrounded by water lettuce. Kruger National Park, South Africa. (This image may be licensed either as rights managed or royalty free.)

Tony Heald / RF- Hippopotamus (Hippopotamus amphibius) coming to end of a yawn whilst in river. Mana Pools National Park, Zimbabwe. October 2012. (This image may be licensed either as rights managed or royalty fre...

Edwin Giesbers / RF- Hippopotamus (Hippopotamus amphibius) captive. (This image may be licensed either as rights managed or royalty free.)

Edwin Giesbers / RF- Hippopotamus (Hippopotamus amphibius) mother and baby, Queen Elizabeth National Park, Uganda. (This image may be licensed either as rights managed or royalty free.)

Andy Rouse / RF- Hippopotamus (Hippopotamus amphibius) aggressive threat display. Katavi National Park, Tanzania. (This image may be licensed either as rights managed or royalty free.)

Andy Rouse / RF- Hippopotamus (Hippopotamus amphibius) threat display, South Luangwa National Park, Zambia. (This image may be licensed either as rights managed or royalty free.)

Tony Heald / RF- Hippopotamus (Hippopotamus amphibius), Moremi Wildlife Reserve, Botswana. (This image may be licensed either as rights managed or royalty free.)

Tony Heald / RF- Two Hippopotamus (Hippopotamus amphibius) play fighting, Chobe National Park, Botswana. (This image may be licensed either as rights managed or royalty free.)

Gabriel Rojo / RF- Hippotamus (Hippopotamus amphibius) submerged in waterhole, Kruger National Park, South Africa. (This image may be licensed either as rights managed or royalty free.)

Tony Heald / RF- Hippopotamus with mouth open (Hippopotamus amphibius). Chobe National Park, Botswana. (This image may be licensed either as rights managed or royalty free.)